RF-Sputtered ZnO Nano-Coatings on Polyamide Thin-Film Composite Membranes for Tuned Nanofiltration Selectivity

Water stress is intensifying worldwide, increasing the need for efficient desalination and water purification technologies. Although commercial nanofiltration membranes such as NF90 exhibit high separation performance, their transport properties remain governed by permeability–selectivity trade-offs, and their surface characteristics offer limited tunability for application-specific requirements. Here, a commercial NF90 polyamide thin-film composite nanofiltration membrane was surface modified by depositing ultrathin ZnO coatings via RF sputtering (30–120 s) and evaluated in terms of surface properties, water permeate flux, and NaCl rejection. X-ray diffraction confirmed the formation of crystalline Wurtzite ZnO with preferential (002) orientation. ZnO deposition markedly increased surface hydrophobicity, raising the water contact angle from 52.5 ± 2.0° for the unmodified membrane to 140.4 ± 3.9° after 120 s of deposition. Hydraulic performance decreased after modification, with water permeate flux reduced by approximately 47–50% relative to pristine NF90. In contrast, NaCl rejection increased with ZnO deposition time, particularly at lower operating pressures, and tended to plateau at higher pressures. The Spiegler–Kedem model accurately described experimental rejection-flux behavior. Overall, RF sputtering of ZnO is a feasible post-fabrication route to tune NF membrane selectivity, while introducing a clear trade-off with permeate flux.